1. Field of the Invention
The present invention generally relates to the fabrication and design of semiconductor chips and integrated circuits, and more particularly to a method of modeling the operation of a circuit running under asynchronous conditions.
2. Description of the Related Art
Integrated circuits are used for a wide variety of electronic applications, from simple devices such as wristwatches, to the most complex computer systems. A microelectronic integrated circuit (IC) chip can generally be thought of as a collection of logic cells with electrical interconnections between the cells, formed on a semiconductor substrate (e.g., silicon). An IC may include a very large number of cells and require complicated connections between the cells. A cell is a group of one or more circuit elements such as transistors, capacitors, resistors, inductors, and other basic circuit elements grouped to perform a logic function. Cell types include, for example, core cells, scan cells and input/output (I/O) cells. Each of the cells of an IC may have one or more pins, each of which in turn may be connected to one or more other pins of the IC by wires. The wires connecting the pins of the IC are also formed on the surface of the chip. For more complex designs, there are typically at least four distinct layers of conducting media available for routing, such as a polysilicon layer and three metal layers (metal-1, metal-2, and metal-3). The polysilicon layer, metal-1, metal-2, and metal-3 are all used for vertical and/or horizontal routing.
An IC chip is fabricated by first conceiving the logical circuit description, and then converting that logical description into a physical description, or geometric layout. This process is usually carried out using a “netlist,” which is a record of all of the nets, or interconnections, between the cell pins. A layout typically consists of a set of planar geometric shapes in several layers. The layout is then checked to ensure that it meets all of the design requirements, particularly timing requirements. The result is a set of design files known as an intermediate form that describes the layout. The design files are then converted into pattern generator files that are used to produce patterns called masks by an optical or electron beam pattern generator. During fabrication, these masks are used to pattern a silicon wafer using a sequence of photolithographic steps. The process of converting the specifications of an electrical circuit into a layout is called the physical design.
Cell placement in semiconductor fabrication involves a determination of where particular cells should optimally (or near-optimally) be located on the surface of a integrated circuit device. Due to the large number of components and the details required by the fabrication process for very large scale integrated (VLSI) devices, physical design is not practical without the aid of computers. As a result, most phases of physical design extensively use computer-aided design (CAD) tools, and many phases have already been partially or fully automated. Automation of the physical design process has increased the level of integration, reduced turn around time and enhanced chip performance. Several different programming languages have been created for electronic design automation (EDA), including Verilog, VHDL and TDML. A typical EDA system receives one or more high level behavioral descriptions of an IC device, and translates this high level design language description into netlists of various levels of abstraction.
Faster performance and predictability of responses are elements of interest in circuit designs. As process technology scales to the deep-submicron (DSM) regime, clock-related problems such as clock skew (jitter) and worst-case execution time are becoming increasingly important to the performance and reliability of IC chips and systems. Asynchronous circuits are often used in situations where such clock-related problems cannot be tolerated, but asynchronous circuit designs are difficult to test. Consequently, modeling of asynchronous circuits has become crucial to achieving an accurate system analysis, particularly the modeling of asynchronous connections between multiple synchronous clock domains (asynchronous boundaries).
With synchronous logic, static timing is performed to ensure that when a latch transitions, the correct value will meet the timing requirements of any downstream latch. One clock cycle is enough time for the transitioning value to be seen on the latch input without violating the setup requirements for that latch. Unfortunately, with asynchronous connections it is unrealistic to maintain static timing requirements because the receive latch may be clocked at any time after the send latch transitions. The transitioning data may not have had enough time to reach the input of the receive latch, and if the new value of the send latch fails to reach the receive latch prior to its sampling of the input, the prior value will the latched. If the transition occurs within the setup time required by the receive latch, the latch may become metastable. For a receive clock period, an old (pre-transition) value or new (post-transition) value may be latched, or the latch may be metastable for that clock period.
One technique for modeling asynchronous behavior is to substitute or insert additional logic in the behavioral description, netlist or representative circuit model. In one approach, a value for the input of the receive latch is toggled, or randomly chosen, whenever a transitioning value is to be latched. Another approach inserts delay elements in the asynchronous crossing and chooses a random delay for each crossing which is fixed for a duration of time. One limitation of the first approach is that a small window of time is created when synchronized transitioning data placed on an asynchronous bus will yield non-deterministic/unsynchronized bus values that are latched. The latter approach requires randomizer logic and delay logic as well as a multiplexer to choose the propagation delay to emulate, which can be quite expensive in terms of the amount of logic required for the modeling process. These requirements are especially costly if many asynchronous crossings exist in the model in which this logic will be inserted.
Another limitation of most transformations is that the netlist is transformed at the receive end of an asynchronous boundary. This method may exclude modeling some of the asynchronous behavior which occurs in crossings with combinational logic. An example is an asynchronous crossing between two latches with the source latch driving both inputs of an XOR gate, and the XOR gate output feeding the input to the receive latch. Asynchronous problems are producible with this logic configuration only if the model is transformed from the send side of the asynchronous crossing since the output from the XOR gate will otherwise never transition. Send-side skewing will only produce the output glitch when each input of the XOR is driven separately, i.e., each input of the XOR gate is driven by separate skew logic. However, the optimal skew logic for this example should ideally be able to produce all possible XOR output results. Transformations from the send side of asynchronous crossings have been devised but those transformations rely on the clock signal of the send latches to determine the starting time of skewing, which makes them inapplicable to some driving elements such as primary inputs. It would, therefore, be desirable to devise an improved method of circuit modeling which used simpler data skew logic transformations to emulate asynchronous behavior. It would be further advantageous if the method did not need to rely on clock signals in any model transformation, and were thus applicable to primary inputs as well as latches.